In order to enable the use of broadband multimedia services, such as for example digital distribution television, via multimedia communication networks, individual programs to be transmitted to subscribers or requested video data ("video on demand", "multimedia on demand") are transmitted to individual virtual connections of an ATM-oriented (Asynchronous Transfer Mode) multimedia communication network. For the selection and reception of the digital video data, each subscriber requires a network terminal unit connected to the multimedia communication network. The unit is known to those skilled in the art as a "set-top box," to which a conventional TV device or a multimedia terminal apparatus can be connected via a conventional unidirectional interface (for example a SCART interface (Syndicat des Constructions d'Appareled Radio, Recepteurs et Televiseurs), a conventional TV set or a multimedia terminal apparatus). The selection of a current program or of the desired digital video data ensues via the remote operation of the set-top box. In this box, a protocol message is formed that indicates the selected video data, and is transmitted via a subscriber terminal unit to a network control unit arranged in the multimedia communication network. This control unit manages the allocation of the available resources or bandwidth between the subscriber terminal unit and the network control unit. In order to ensure the optimal use of the available resources or bandwidth, that is to avoid redundancy in the transporting of the video data, only the current programs or digital video data requested via the set-top boxes are distributed to the respective subscriber terminal unit by the network control unit. If a set-top box requests a program or digital video data, the bandwidth or, respectively, free virtual connection required for the transmission of the digital video data is reserved or, respectively, allocated by the network control unit, insofar as the requested program or, respectively, digital video data are not already transmitted for another set-top box. If an end of the requested video data transmission is signaled by the set-top box, the allocated resources (that is, the virtual connection reserved therefor) are again released, provided the relevant digital video data is not to be transmitted for any additional set-top box.
In the releasing of resources, that is the bandwidth of the allocated digital video data, the following problem now arises. If a subscriber switches off the TV set connected to the set-top box (for example, switching into standby mode via remote control) without communicating the termination of the requested video data transmission to the set-top box, the set-top box cannot transmit the end of the video data request to the network control unit. If no other set-top box receives the same program or the same digital video data, the allocated bandwidth remains allocated for the now switched-off TV set, whereby the allocated virtual connection remains unused. in the extreme case, the switching-oriented resources and transmission-oriented resources between the network control unit and the subscriber terminal unit are blocked by virtual connections that are allocated but are not being used.